The invention relates to tire molds, and in particular to tire molds with special inserts for air vent openings or holes passing through the molds.
Molds for rubber tires are usually provided with a plurality of strategically located small diameter passages from the cavity to the mold exterior to bleed or vent trapped air from the mold during use so that contact between the rubber tire being formed in the mold and the cavity wall is optimized. The vent holes are normally machined through the walls of the mold by drilling, or the like. In forming these holes, there is a minimum hole diameter attainable due to structural characteristics of the tire mold as well as limitations of the machining operations and equipment.
It is advantageous to provide as small a diameter vent hole as will still permit air venting but will reduce the amount of and opportunity for rubber to flow into the hole which ultimately forms as projections from the surface of the finished tire. These projections, depending on their size and thickness, must be removed from the finished tire and/or can remain in the vent hole by being torn from the tire surface upon removal of the tire from the mold thereby plugging or closing the vent hole for subsequent operation.
To reduce the hole diameters beyond what is normally achievable by machining, cylindrical metal inserts or "plugs" with very small openings or bores therethrough have been fitted into the cavity side portions of the machined vent holes, thus providing the desired small openings (see U.S. Pat. No. 3,377,662). It has been further found that to provide such inserts made from a low friction material, such as plastic, is even more advantageous, in reducing the possibility of rubber projections remaining in the insert by tearing from the tire upon removal. Because these low friction plastic inserts offer less resistance or are non-adherent to rubber, the projections more often slide clear from the insert bore rather than tear (see U.S. Pat. No. 3,377,662 and 3,804,566).
Using these plastic inserts, however, have caused certain problems different from those minimized by use of such inserts. Such plastic inserts have been found to "creep" or "swell" while in use causing closure of the vent hole and/or causing the plugs to project into the cavity leaving a marred or blemished appearance on the surface of the finished article.